The present invention relates to semiconductor lasers, and more particularly, to the fabrication of mirror elements in such lasers.
Semiconductor lasers are widely used in optical disk drives and the like to write or read data. Since the density of data that can be stored on optical disks depends on the wavelength of the laser used to read and write the data, semiconductor lasers for generating light at short wavelengths are actively being pursued. Lasers based on Group III-V lasers emit light in the blue wavelengths; however, commercialization of this technology has been hampered by a number of problems.
There are two general types of semiconductor lasers, edge-emitting lasers and surface-emitting lasers. In edge-emitting lasers, the laser cavity is formed by utilizing cleaved facets to form the mirrors that define the edges of the cavity. Light exits the laser in a direction parallel to the substrate surface. While edge-emitting lasers are relatively easy to produce in single quantities, this technology cannot be easily utilized to form two-dimensional arrays of lasers. In addition, the cross-section of the laser generated light beam is rectangular, whereas circular beam cross-sections are preferred.
In contrast, surface-emitting lasers emit light in a direction perpendicular to the surface of the substrate. This type of laser lends itself to the construction of two-dimensional arrays. In addition, the cross-section of the laser beam can be controlled using conventional masking techniques, and hence, beams having circular cross-sections are easily obtained.
Unfortunately, the construction of surface-emitting lasers is much more complex than edge-emitting lasers. In surface-emitting lasers, the mirrors are deposited as layers that are parallel to the substrate surface. While a number of technologies can be utilized to form the mirrors, semiconductor multi-layer film methods are preferred because the generation of the multilayer film utilizes the same equipment and process steps as the other layers in the laser structure. For example, a multilayer film reflective mirror composed of GaN layers and In AlN layers is disclosed in Unexamined Japanese Patent Publication No. HEI 7-297476. Unexamined Japanese Patent Publication No. HEI 7-335975 discloses a multilayer reflective mirror composed of two types of AlGaN layers having different composition ratios of Al, Ga, and N.
While these Group III-V nitride semiconductor surface-emitting lasers have advantages over edge-emitting lasers as discussed above, extending the lifetime, decreasing the threshold current, obtaining higher power output, and lowering manufacturing costs are needed. Significant progress toward these goals could be achieved if the number of layers in the mirrors could be reduced and the size and density of defects in the mirror layers could also be reduced.
The number of layers required to fabricate a mirror of a given reflectivity depends on the ratio of the indices of refraction of the layers. As noted above, the mirrors are formed by cyclically depositing layers of different indices of refraction. Typically, a repeated two-layer structure is utilized. The layers differ in index of refraction; hence, light will be partially reflected at the interface of the layers. The fraction of the light that is reflected at each interface depends on the ratio of the indices of refraction, greater ratios providing higher reflectivity. The thickness of the layers is chosen such that the light reflected from each boundary will add coherently. Hence, multilayer structures having the greatest difference in refractive index are preferred because fewer layers are needed.
Unfortunately, in prior art devices, the size and density of defects in the layers increases as the index of refraction ratio increases when Group III-V nitride semiconductors are utilized. In these material systems, materials with substantially different indices of refraction also have substantially different lattice constants. The lattice constant mismatch between the layers leads to defects in the layers. Hence, designers have been faced with a tradeoff between the number of mirror layers and the defect density in the mirror layers. If layers having small lattice constant mismatches are used to reduce defect density, a larger number of layers must be utilized to obtain the desired reflectivity. The increased number of layers leads to increased fabrication costs and higher threshold currents. The higher threshold currents also lead to increased heat generation and a reduction in device lifetime. If layers having high differences in index of refraction are used, defect densities increase, cracks are generated and device yield decreases leading to higher device cost.
A number of methods for reducing defects have been reported. For example, in epitaxial lateral overgrowth (ELOG), a GaN layer is grown at a high temperature on the substrate. A SiO2 layer is in strips having widths of several microns on the surface of the GaN layer. Then GaN is grown over the SiO2 strips by seeding further epitaxial growth from the GaN areas between the strips. The GaN that is formed over the SiO2 in this manner is substantially defect-free except for a defect at the point at which the overgrowth from the two sides of the SiO2 strip meets. Unfortunately, this method does not lend itself to the formation of the mirror layers described above.
A second method for reducing defects is referred to as the double buffer method. In the double buffer method, an amorphous GaN layer or AlN layer is grown to a thickness of several dozen nanometers on the substrate. A flat layer is then formed on top of this layer by growing a crystalline GaN layer having a thickness of several microns. This growing process is repeated twice to provide a GaN layer with reduced levels of defects. The double buffer technique is disclosed in the specification of Japanese Patent Application No. HEI 9-30215.
Broadly, it is the object of the present invention to provide an improved method for fabricating mirror layers in semiconductor lasers.
It is a further object of the present invention to provide a method for fabricating mirror layers that allows group III-V materials with substantially different lattice constants to be utilized while maintaining acceptable defect densities.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a mirror for use in a semiconductor laser. The mirror consists a total of four or more alternating layers of a low refractive index semiconductor material having an index of refraction of n1 and a high refractive index semiconductor material having an index of refraction of n2 where n2 greater than n1. A first one of the layers is grown at a temperature, referred to as the xe2x80x9clow temperaturexe2x80x9d, such that the layer is in a substantially amorphous state, and a second one of the layers is grown at a temperature, referred to as the xe2x80x9chigh temperaturexe2x80x9d, such that the layer is in a substantially monocrystalline state. The first one of the layers and the second one of the layers correspond to layers having different indices of refraction. The thickness, TL, of the layer grown at the low temperature satisfies
TL={xcex/(4xc3x97nL)}(2Mxe2x88x921) less than 0.1 xcexcm
where xcex is the wavelength of the light emitted by the laser, nL is the refractive index of the layer grown at the low temperature, and M is a positive integer. The thickness, TH, of the layer grown at the high temperature satisfies
TH={xcex/(4xc3x97nH)}(2Nxe2x88x921) greater than 0.2 xcexcm
where nH is the refractive index of the layer grown at the high temperature, and N is a positive integer. The first and second materials are preferably Group III-V nitrides such as AlxGal-xN, where x=0.3 to 1, and GaN.